Document developer guide on how sunpy-soar works and how to develop

[hayesla]

As discussed on our weekly call, @NucleonGodX has done some excellent work as part of the GSOC to add more functionality to sunpy-soar. It would be great to now add some developers guide to our docs about how sunpy-soar actually works and queries the SOAR to make a Fido search, and how someone could add new functionality to the future etc.

[NucleonGodX]

I'll soon start working on it!

[nabobalis]

Closed by #147